A mother may literally have her children on her mind at all times. Findings reveal that cells from fetuses migrate into the brains of their mothers, and can last a lifetime.

Few would argue that the mother-baby bond during pregnancy is the strongest human connection possible. During pregnancy, a mother is so connected physically and psychologically to her child, that her baby depends on her for everything from nutrition, to blood flow to warmth and more. But baby also provides mother with some special.

Recent findings showed that during pregnancy, mothers and fetuses often exchange cells that can apparently survive in bodies for years, a phenomenon known as microchimerism. Scientists had found that in mice, fetal cells could even migrate into the brains of mothers. Now researchers have the first evidence fetal cells do so in humans as well.

Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades or lifetime. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver.

In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants.

A fetal microchimeric cell from a pregnancy is recognized by the mother’s immune system partly as belonging to the mother, since the fetus is genetically half identical to the mother, but partly foreign, due to the father’s genetic contribution. This may “prime” the immune system to be alert for cells that are similar to the self, but with some genetic differences.

Cancer cells which arise due to genetic mutations are just such cells, and there are studies which suggest that microchimeric cells may stimulate the immune system to stem the growth of tumors. Many more microchimeric cells are found in the blood of healthy women compared to those with breast cancer, for example, suggesting that microchimeric cells can somehow prevent tumor formation.

In other circumstances, the immune system turns against the self, causing significant damage. Microchimerism is more common in patients suffering from Multiple Sclerosis than in their healthy siblings, suggesting chimeric cells may have a detrimental role in this disease, perhaps by setting off an autoimmune attack.

Investigators analyzing the brains of 59 women who had died between the ages of 32 and 101, looked for signs of male DNA --which, they reasoned, would have come from the cells of sons. (They searched for male DNA because female DNA would have been harder to distinguish from a mother's genes.)

Nearly two-thirds of the women -- 37 of the 59 -- were found to have traces of the male Y chromosome in multiple regions of their brains. This effect was apparently long-lasting: The oldest female in whom male fetal DNA was detected was 94.

"In the fetus, we found that there is an immune system whose job it is to teach the fetus to be tolerant of everything it sees, including its mother and its own organs," said Joseph M. McCune, MD, PhD, a professor in the UCSF Division of Experimental Medicine. "After birth, a new immune system arises from a different stem cell that instead has the job of fighting everything foreign."

The defense system known as the blood-brain barrier keeps many drugs and germs in the bloodstream from entering the brain. However, doctors have found this barrier becomes more permeable during pregnancy, which could explain how these fetal cells migrated into the brains of their mothers.

"The most important implication of our findings is the potential for both positive and negative consequences of microchimerism in the brain for a number of different diseases that affect the brain, including degenerative diseases and cancer," researcher William Chan, an immunologistat the Fred Hutchinson Cancer Research Center in Seattle stated.

This is a burgeoning new field of inquiry with tremendous potential for novel findings as well as for practical applications. But it is also a reminder of our interconnectedness.